As the angular velocity sensor of the vibration type, there has been proposed the one provided with a base portion, a vibrator coupled to the base portion, exciting means for driving and vibrating the vibrator in a first direction, and detection means for detecting the angular velocity based on the vibration of the vibrator in a second direction at right angles with the first direction when the angular velocity is applied thereto while the vibrator is being driven and vibrated (see, for example, JP-A-2003-42767).
FIG. 4 is a diagram of a general angular velocity sensor 800 of this kind inclusive of a circuit portion 900.
The angular velocity sensor 800 illustrated in FIG. 4 comprises a semiconductor substrate 10 such as a silicon substrate in which trenches are formed relying on a known semiconductor production technology such as etching, thereby to form base portions 20 on the outer peripheral portion thereof, and to form two vibrators 30 and 40 as well as electrodes on the inner peripheral portion thereof in a sectionalized manner as shown in FIG. 1.
Here, the angular velocity sensor 800 has the vibrator that is constituted by the first vibrator 30 and the second vibrator 40 movably coupled to the base portions 20.
Further, the circuit portion 900 is formed on another IC chip that is not shown, or is the one obtained by integrally forming elements such as transistors in the semiconductor substrate 10 that constitutes the angular velocity sensor 800 relying upon the semiconductor production technology. The connection of the circuit portion 900 to base portions 20, vibrators 30, 40, and electrodes, can be accomplished by using the bonding wires or various wiring members.
The first vibrator (lower side in the drawing) 30 and the second vibrator (upper side in the drawing) 40 located on the inner periphery of the base portions 20, are arranged along the x-direction (first direction) in FIG. 1, and are both movable relative to the base portions 20.
The two vibrators 30 and 40 include frame portions 31, 41, rectangular portions 32, 42 of nearly a rectangular shape positioned on the inside of the frame portions 31, 41, and detection beams 33, 43 for coupling the frame portions 31, 41 to the rectangular portions 32, 42. The two vibrators 30, 40 are supported at their frame portions 31, 41 by being coupled to the base portions 20 through drive beams 50.
The drive beams 50 have a freedom in the x-direction (first direction) in FIG. 1, and the detection beams 33, 43 have a freedom in the y-direction (second direction) in FIG. 1.
The first vibrator 30 and the second vibrator 40 have exciting means provided on both sides of the vibrators 30, 40 along the x-direction for generating electrostatic force for driving and vibrating the two vibrators 30, 40 in the x-direction in opposite phases to each other.
Here, the exciting means are constituted by drive electrodes 34, 44, amplifiers 811, 812, 813, 814, and a drive signal generating circuit 820 in the circuit portion 800.
In the exciting means, the drive voltage is applied to the drive electrodes 34, 44 from the drive signal generating circuit 820 and the amplifiers 811 to 814. The drive voltage is an AC voltage such as of a sine wave or a rectangular wave.
The vibrators 30 and 40 are constituted so that AC voltages of opposite phases are applied to the drive electrodes 34, 44 of both sides. Namely, the vibrators 30 and 40 are driven and vibrated in the x-direction (drive direction x) in opposite phases to each other due to the electrostatic forces.
Referring to FIG. 4, further, detection means are provided for the vibrators 30 and 40 being coupled to the base portions 20 for detecting the angular velocity. The detection means are constituted by detection electrodes 60,70, and C/V converters 931, 932, 933, 934 in the circuit portion 900.
When an angular velocity is applied about the z-axis (detection axis) in FIG. 4 while the vibrators 30, 40 are being driven and vibrated, the vibrators 30 and 40 vibrate in the y-direction (detection direction y) at right angles with the x-direction due to the Coriolis' force. Due to the vibration of the vibrators 30 and 40 in the y-direction, the capacitances vary between the detection electrodes 60, 70 and the rectangular portions 32, 42 of the vibrators 30, 40 facing the detection electrodes 60, 70. Variations in the capacitances are converted into voltages through the C/V converters 931 to 934 and are output.
Here, a differential output between the two C/V converters 931 and 932 in the first vibrator 30, is obtained and, further, a differential output between the two C/V converters 933 and 934 in the second vibrator 40, is obtained. Further, a difference between these two differential outputs is obtained and is output as an angular velocity signal. Therefore, due to the first and second vibrators 30 and 40 driven and vibrated in opposite phases relative to each other, there is obtained an angular velocity signal canceling the acceleration component in the y-direction.
Referring to FIG. 4, the circuit portion 900 of the angular velocity sensor 800 is provided with constant potential portions 940 maintaining the vibrators 30, 40 at predetermined potentials. The constant potential portions 940 are electrically connected to the base portions 20 that are supporting the frame portions 31, 41 of the vibrators 30, 40.
Here, if the above angular velocity sensor 800 is used for detecting the angular velocity about the vertical axis, i.e., for detecting the yaw rate, the drive direction x of the vibrators 30, 40 and the detection direction y on which the Coriolis' force acts are set on a horizontal plane to be perpendicular to each other. Therefore, the vibrators 30 and 40 are driven and vibrated uniformly toward the right and left.
When the angular velocity sensor 800 is used for detecting, for example, the angular velocity about the axis in the back-and-forth direction, i.e., roll rate and the angular velocity about the axis in the right-and-left direction, i.e., pitch rate other than the yaw rate, the detection axis (the above z-axis) about which the angular velocity occurs is on the horizontal plane. Therefore, either the drive direction x or the detection direction y becomes in agreement with the direction of the gravity.
Then, the vibrators 30 and 40 are placed in a state where the gravity 1G is added to the direction of vibration at all times. Therefore, the vibrators 30 and 40 are driven and vibrate in a state of being pulled by the gravity 1G in either one of the vibrating directions. Namely, the drive beams are differently deformed by the right and left vibrations, and the state of vibration becomes nonuniform.
Further, when the detection direction y is in agreement with the direction of the gravity, the vibrators 30 and 40 are deviated at all times from the geometrical centers, i.e., deviated from the positions of the vibrators of when they are stationary and free from the gravity. Therefore, the detection beams are deformed, and the driving vibration leaks in the detection direction y accounting for a cause of noise. Here, however, since a difference in the outputs of the detection electrodes is taken out, the noise in principle can be canceled.